The development of laboratory methods to detect persons with a genetic susceptibility to breast cancer could aid in the determination of genetic and environmental interactions which result in disease and the development of preventive measures. This study tests whether variations in peripheral lymphocyte mutagen sensitivity, as determined by cytogenetic analysis, are associated with the diagnosis of breast cancer or the elevated breast cancer risk of women with atypical hyperplasia and a family history of breast cancer. Lymphocyte cultures from 20 newly diagnosed untreated patients with invasive breast cancer, 40 high risk breast cancer risk women and 40 control subjects will be established and the cells treated with a battery of mutagens with widely different mechanisms of action. The resultant cytogenetic damage will be assessed using both conventional chromosomal aberration studies and the analysis of sister chromatid exchange (SCEs). The use of a battery of mutagens and multiple endpoints of cytogenetic damage increases the power of the study to detect associations of breast cancer risk and mutagen sensitivity. The concurrent study of high breast cancer risk individuals and women with frank cancer will further indicate the extent to which these measurements of mutagen sensitivity are affected by neoplasia.